Flexible printing filaments, typically made from TPE and TPU are thermoplastic elastomers that allow the print to stretch or bend easily. 

TPE (thermoplastic elastomer) is a blend of hard plastic and soft rubber, so it has both thermoplastic and elastic properties. TPE includes a wide range of flexible materials including thermoplastic polyurethane (TPU), thermoplastic copolyester (TCP), thermoplastic polyamide (TPA)

TPU (thermoplastic polyurethane) is the most common type of TPE and is a more rigid flexible filament, making it easier to print.